1. Field of the Invention
The present invention relates to a vehicle seat having a headrest.
2. Description of the Related Art
There has been proposed a headrest drive capable of moving a headrest forward to hold an occupant's head when a vehicle is bumped from behind. For example, Jpn. Pat. Appln. KOKAI Publication No. 11-268566 (patent document 1) discloses a conventional headrest drive, in which a connection member is arranged in the lower part of a bracket to support a stay of a headrest. The connection member is provided with a pressure-receiving member. In this conventional example, when an occupant is pressed to a seat back upon a collision, the pressure-receiving member is pressed rearward, the connection member is driven, and the headrest is moved forward.
Jpn. Pat. Appln. KOKAI Publication No. 2001-58533 (patent document 2) discloses another conventional headrest drive, in which a pressure-receiving member is arranged above an S-shaped spring of a seat back. In this conventional example, the pressure-receiving member is fixed to the lower part of a headrest holder bracket. When a vehicle is bumped from behind, the pressure-receiving member is pushed rearward by the load applied to a seat back from an occupant, and a headrest is moved forward.
It is known that when a vehicle is bumped from behind, a vehicle seat having a headrest drive behaves as shown in FIG. 7. In FIG. 7, the characteristic curve A drawn by a solid line indicates rearward movement (X-amount) of an occupant's head with time. The characteristic curve B drawn by a broken line indicates rearward movement (X-amount) of a headrest with time. At a point of intersection C of the characteristic curves A and B, the occupant's head contacts the headrest.
In a vehicle seat having a headrest drive, a characteristic curve includes a flat part F (shown in FIG. 7) showing a relatively small increase in the X-amount of a headrest. The time taken by the occupant's head to contact the headrest (the time from a collision to the point C) can be shortened by the flat part F.
Jpn. Pat. Appln. KOKAI Publication No. 2007-106378 (patent document 3) discloses another vehicle seat, which has a connection member to connect the upper part of a seat back frame to the upper part of a plane spring structure, and a shock-absorbing part provided in the connection member. According to the patent document 3, a vehicle seat facilitates rearward movement of an occupant upon a collision from behind.
In the vehicle seat with a headrest drive incorporated in a seat back as described in the patent documents 1 and 2, the weight of a seat is increased by the weight of a headrest drive. The structure of a headrest drive is complicated, and the seat cost is increased.
In the vehicle seat described in the patent document 3, a shock-absorbing part is provided in a connection member provided in the upper part of a seat back. In this structure, the shock-absorbing part cannot function effectively, when a relatively large load is applied to the lower half part of a seat back from the part around an occupant's waist upon a collision from behind. Namely, when a seat back is moved rearward by the load applied to a seat back, a headrest is also moved rearward, and a head support function equivalent to the movable headrest drive is not obtained.